A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. A circuit breaker automatically has to open its contacts if an overcurrent condition is sensed. Therefore, a circuit breaker comprises a trip unit which determines when the contacts have to open.
Some circuit breakers comprise a thermo magnetic trip unit. The thermo magnetic trip unit includes elements designed to sense the heat resulting from an overload condition and the high current resulting from a short circuit. In addition, some circuit breakers incorporate a “push to trip” button.
It is possible that a temperature profile of a thermo magnetic trip unit of a standard circuit breaker does not meet the requirements of each circuit breaker. For example, increasing the temperature inside the thermo magnetic trip unit of a first circuit breaker can generate higher temperatures on the lugs that do not comply with temperatures on the lugs of standard circuit breakers.
A low temperature profile can generate low temperatures in the bimetal of a thermo magnetic trip unit which result in a low bimetal deflection and a low power development of the bimetal. Although the deflection of the bimetal can be compensated by using a calibration screw to get the bimetal closer to the trip bar of the thermo magnetic trip unit, the temperature may not be enough for the bimetal to produce enough power to rotate the trip bar and release the energy storage spring “kicker” of the thermo magnetic trip unit.
For example, familiar circuit breakers comprise a thermo magnetic trip unit 1 with braid plate 2, a load plate 3, a heater 4 and a bimetal 5 which has an indirect heating through the heater 4. The bimetal 5 can be screwed to the heater 4 and/or a heater support plate 8. A trip bar 6 of the thermo magnetic trip unit 1 rotates as soon as it is moved by the bimetal 5. The trip bar 6 can release the energy storage spring 7 of the thermo magnetic trip unit 7 to open the contacts of the circuit breaker. A thermal calibration screw 9 can be used to calibrate the thermo magnetic trip unit 1 to increase or decrease the time the bimetal 5 needs to touch the trip bar 6, see FIGS. 1 to 4.
FIG. 2 shows a familiar thermo magnetic trip unit 1 with a bimetal 5 in its normal position. If no current is flowing through the current path of the thermo magnetic trip unit 1, the bimetal 5 is in a straight “normal” position.
If sufficient overcurrent flows through the circuit breaker's current path, heat build-up causes the bimetal 5 of the thermo magnetic trip unit 1 to bend. As the bimetal 5 is heated, it bends from its high expansion side to its low expansion side. After bending a predetermined distance, the bimetal 5 touches the trip bar 6 activating the energy storage spring 7 and thus the trip mechanism of circuit breaker. If 120% of the breaker nominal current flows through the current path, the bimetal 5 generates power according to the available temperature which rotates the thermal trip bar 6 and releases the energy storage spring 7, see FIG. 3.
If the temperature is not enough, the bimetal 5 may bend and touch the trip bar 6, but it won't generate enough power to rotate the trip bar 6, see FIG. 4. If 100% of the current flows through the current path of the thermo magnetic trip unit 1 the bimetal 5 can bend but can fail to touch the trip bar 6.